Current stabilizing circuit arrangement

ABSTRACT

A current stabilizing arrangement includes a first circuit having a series arrangement of a first resistor, a second resistor, and the collector-emitter path of a first transistor having its base connected to a point between the first and second resistors. A second circuit includes the collector-emitter path of a second transistor whose base is coupled to the collector of the first transistor. By providing a third resistor in the first circuit, in series with the first and second resistors and connected between the base of the second transistor and the collector of the first transistor, improved current stabilization with variations in supply voltage is obtained.

BACKGROUND OF THE INVENTION

The invention relates to a current stabilizing arrangement comprising afirst circuit between a first and a second power-supply terminal whichcomprises a series arrangement of a first resistor, a second resistor,and the collector emitter path of a first transistor whose base isconnected to a point between the first and the second resistor, and asecond circuit between a third terminal and the second power-supplyterminal which comprises the collector-emitter path of a secondtransistor of the same conductivity type as the first transistor, whosebase is coupled to the collector of the first transistor.

Such an arangement is suitable for general use in integrated circuits.In particular, such a circuit arrangement may be used in a one-chipintegrated radio receiver.

Such a circuit arrangement is known from U.S. Pat. No. 3,831,040. Inthis arrangement the current in the first circuit is the unstabilizedcurrent and the current in the second circuit is the stabilized current.Stabilization is achieved by having the current in the first circuit,which can be adjusted by means of the first resistor, produce asubstantially constant voltage across the first transistor which isarranged as a diode. In order to ensure that the current in the secondcircuit is also stabilized with respect to supply-voltage variations, asecond resistor is arranged between the base and the collector of thefirst transistor, the base of the second transistor being connected tothe collector of the first transistor. In the case of a supply-voltagevariation, the voltage variation across the first transistor which isarranged as a diode is substantially equal to the voltage variationacross the differential resistance of the diode. In order to make thecurrent in the second circuit independent of these last-mentionedvoltage variations, the voltage across the differential resistance iscompensated for by the voltage across the second resistor.

However, the differential resistance of a diode is inverselyproportional to the current through the diode. For a specific value ofthe second resistor this means that the voltage variation across thesecond resistor is equal to the voltage variation across thedifferential resistance for only one specific current and, consequently,one specific supply voltage. The current in the second circuit istherefore independent of supply-voltage variations only to a limitedextent. In the case of a suitable value of the second resistor, theknown circuit arrangement enables the current in the second circuit tobe stabilized to within 5% in the voltage range of approximately 2 to 10V, which is the customary range for integrated circuits.

SUMMARY OF THE INVENTION

It is the object of the invention to provide a current stabilizingarrangement which is more independent of supply-voltage variations. Acurrent stabilizing arrangement of a type as set forth in the openingparagraph is characterized in that in the first circuit, in series withthe first and the second resistor, a third resistor is arranged betweenthe connection point of the base of the second transistor and thecollector of the first transistor. The third resistor limits the voltagevariation across the second resistor to a maximum value which isdetermined by the ratio between the resistance values of the second andthe third resistors. The third resistor can now ensure that the voltagevariation across the second resistor is substantially equal to thevoltage variation across the differential resistance over a largevoltage range. A current stabilizing arrangement in accordance with theinvention is characterized in that in the first circuit, in series withthe collector-emitter path of the first transistor, thecollector-emitter path of a third transistor is arranged, whose base iscoupled to its collector, and in the second circuit a fourth resistor isarranged between the emitter of the second transistor and the secondpower-supply terminal.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail, by way of example,with reference to the accompanying drawing, in which;

FIG. 1a shows a known type of current stabilizing arrangement;

FIG. 1b shows current-voltage characteristics of the current stabilizingarrangement shown in FIG. 1a;

FIG. 2a shows a current stabilizing arrangement in accordance with theinvention; and

FIG. 2b shows a current-voltage characteristic of the currentstabilizing arrangement shown in FIG. 2a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows a known type of current stabilizing arrangement using thearrangement described in the aforementioned U.S. Pat. No. 3,831,040.Between two power-supply terminals 6 and 7 the circuit arrangementincludes a first circuit which comprises the series arrangement of afirst resistor 1, a second resistor 2, the collector emitter path of afirst transistor T₁ whose base is coupled to a point between the firstresistor 1 and the second resistor 2, and the collector-emitter path ofa second transistor T₂ which is arranged as a diode. Between thepower-supply terminals 6 and 7 the circuit arrangement further comprisesa second circuit which comprises a load 5, which is shown schematically,the collector-emitter path of a third transistor T₃ whose base iscoupled to the collector of transistor T₁, and a resistor 4. The currentI₂ in the second circuit is substantially equal to I₂ =V_(BE) /R₄,V_(BE) being the base-emitter voltage of transistor T₃ arranged as adiode and R₄ being the value of the resistor 4. In order to ensure thatthe current I₂ supplied to the load 5 by the transistor T₃ is constant,the voltage on the base of transistor T₃ must be constant. The currentI₁ through the first circuit is adjusted by means of the resistor 1. Thevoltage V_(B3) on the base of transistor T₃ is approximately determinedby the formula:

    V.sub.B3 =2V.sub.BE +2I.sub.1 r.sub.0 -I.sub.1 R.sub.2

in which V_(BE) is the base-emitter voltage of the transistors T₁ andT₂, r₀ is the differential resistance of the transistors T₁ and T₂ whichare arranged as diodes, and R₂ is the resistance value of the resistor2. In the case of supply-voltage variations, the current I₁ also varies.The base-emitter voltage V_(BE) of the transistors then remainssubstantially constant. It follows from the above formula that the basevoltage V_(B3), and consequently the current I₂, is constant if thevoltage variation across the resistor 2 is equal to the voltagevariation across the differential resistances, or if R₂ =2r₀. As isknown, the differential resistance of a diode is equal to r₀ =kT/qI₁,where k is Boltzmann's constant, T the absolute temperature and q theelectron charge. For values of R₁ which are not too small relative to r₀the approximation I₁ =V/R₁ is valid, which yields r₀ =kTR₁ /qV. Thismeans that for a specific value of R₂ the voltage variation across thedifferential resistances r₀ is compensated for by the voltage variationacross the resistor R₂ over only a limited range of supply voltages.Therefore, the current I₂ is independent of supply-voltage variationsonly to a limited extent. For a specific value of R₁ the supply-voltagerange within which the current I₂ is substantially independent ofsupply-voltage variations depends on the value R₂ of the resistor 2.This will be explained with reference to FIG. 1b, which shows twocurrent-voltage characteristics, the current I₂ in percent being plottedversus the supply voltage V. For the characteristic I the variation ofthe current I₂ is minimal over an as large as possible supply-voltagerange. For this purpose the value of R₂ is selected so that the voltagedrop across R₂ is substantially equal to the voltage drop across thedifferential resistances 2 r₀, which have a value corresponding tosubstantially the center of the voltage range over which the current I₂is to be stabilized. Therefore, the characteristic I substantiallycomplies with:

    R.sub.2 /R.sub.1 =2KT/qV with V≈6 Volt

The variation of I₂ over the range from approximately 2 to 10 V is thenapproximately 5%. If the ratio R₂ /R₁ is increased, stabilization iseffected at lower voltages and over a smaller voltage range. Forcharacteristic II, stabilization is effected for voltages betweenapproximately 2 and 5 V. For higher voltages, the voltage variationacross R₂ is substantially higher than the voltage variation across theresistances 2 r₀, which leads to overcompensation so that the variationof the current I₂ in the voltage range from approximately 2 to 10 V issubstantially greater than 5%.

FIG. 2a shows an embodiment of a current stabilizing arrangement inaccordance with the invention. Identical parts bear the same referencenumerals as in FIG. 1a. The current stabilizing arrangement differs fromthe arrangement shown in FIG. 1a in that in series with the resistors 1and 2 a third resistor 3 is arranged between the base connection oftransistor T₃ and the collector of transistor T₁. The resistor 3 limitsthe voltage variation across the resistor 2. It is found that theresistor 3 limits the compensation voltage for the voltage variationacross the differential resistances to a maximum value of substantiallyV_(BE). R₂ /R₃, R₃, being the value of the resistor 3. This precludesovercompensation. By the addition of a resistor 3 of a suitably selectedresistance value R₃ a stability improvement of a factor of 2.5 can beobtained in comparison with the stabilizing arrangement shown in FIG.1a. FIG. 2b shows a current-voltage characteristic for the circuitarrangement shown in FIG. 2a. The variation of I₂ over the range ofapproximately 2 to 10 V is now ±2%.

In addition to the embodiment shown, the invention may be utilized incurrent stabilizing arrangements comprising one instead of twotransistors in the first circuit and with or without a resistor in theemitter line of the transistor in the second circuit. Instead ofNPN-transistors the current stabilizing arrangements in accordance withthe invention may be equipped with PNP-transistors.

What is claimed is:
 1. A current stabilizing arrangement comprising afirst circuit between a first and a second power supply terminal whichcomprises a series arrangement of a first resistor, a second resistorand the collector-emitter path of a first transistor whose base isconnected to a point between the first and the second resistor, and asecond circuit between a third terminal and the second power supplyterminal which comprises the collector-emitter path of a secondtransistor whose base is coupled to the collector of the firsttransistor, characterized in that in the first circuit, in series withthe first and the second resistor, a third resistor is provided betweenthe connection point of the base of the second transistor and thecollector of the first transistor.
 2. A current stabilizing arrangementas claimed in claim 1, characterized in that in the first circuit, inseries with the collector-emitter path of the first transistor, thecollector-emitter path of a third transistor is provided, the base ofsaid third transistor being coupled to its collector, and in the secondcircuit a fourth resistor is provided between the emitter of the secondtransistor and the second power-supply terminal.